Process for making a glass fiber with a core and two glass cladding layers and glass fiber made thereby

ABSTRACT

The glass fiber for an optical amplifier has a glass core, a first glass cladding, and a second glass cladding. The core has a composition, in mol %, of Bi 2 O 3 , 30-60; SiO 2 , 0.5-40; B 2 O 3 , 0.5-40; Al 2 O 3 , 0-30; Ga 2 O 3 , 0-20; Ge 2 O 3 , 0-25; La 2 O 3 , 0-15; Nb 2 O 5 , 0-10; SnO 2 , 0-30; alkali metal oxides, 0-40; and Er 2 O 3 , 0.05-8. The process for making the glass fiber includes first making a preform consisting of the core and the first glass cladding by drawing from a double crucible. Then the second glass cladding is formed around the preform by a rod-in-tube process. The glass claddings have a composition that includes a transition metal compound as an absorbent.

CROSS-REFERENCE

This is a divisional of U.S. patent application Ser. No. 10/489,020,filed on Aug. 16, 2004. The aforesaid U.S. patent application, whosedisclosures are expressly incorporated herein by reference thereto,describes the same invention as described and claimed herein below. Inaccordance with 35 U.S.C. 120 the benefit of the filing date of theaforesaid U.S. patent application is claimed for the claims presentedherein below.

BACKGROUND OF THE INVENTION

1. The Field of the Invention

The present invention relates to a glass fiber which comprises a core,the matrix glass of which contains at least one heavy metal oxide and atleast one rare earth compound, the core being surrounded by at least twoglass claddings. Furthermore, the present invention relates to a processfor producing a glass fiber according to the invention, to an opticalamplifier which comprises at least one glass fiber according to theinvention, and to the use of the glass fiber according to the invention.

2. The Related Art

Optical amplifiers are one of the most important key components ofoptical communication technology. If a purely optical telecommunicationssignal is transmitted in a glass fiber, it is inevitable that intrinsicsignal attenuation will occur. To compensate for this attenuation, it isnecessary to use highly efficient optical amplifiers which are able toamplify a signal without the optical signal having to be converted intoan electronic signal and then back into an optical signal. Opticalamplifiers can also increase the speed of amplification, and thedeterioration in the signal/noise ratio is significantly lower onaccount of the elimination of the conversion, into electronic signalsand back.

In this context, the technical demands imposed on optical amplifiers areincreasing in particular on account of the continuously rising demandfor ever greater bandwidths. Currently, broadband data transmission isrealized using WDM (WDM “wavelength division multiplexing”) technology.Most amplifiers of the prior art operate in the C band (approx. 1528 nmto 1560 nm) and have only a limited broadband capacity, since opticalamplifiers of this type have hitherto been based on Er³⁺-doped SiO₂glasses. Therefore, the demand for greater bandwidths has required thedevelopment of multicomponent glasses, for example heavy metal oxideglasses (HMO glasses). As manifested by their intrinsically very highrefractive index (at 1.3 μm) of n>approx. 1.85, heavy metal oxideglasses have high internal electrical fields and therefore, on accountof greater Stark splitting, lead to broad-band emission from the rareearth ions. However, the high refractive index of HMO glasses also leadsto new problems which have to be overcome.

Various mechanisms in optical amplifier fibers can give rise toscattered light, which can lead to a deterioration in the signal/noiseratio and should therefore be removed or avoided as fully as possible.

In amplifier fibers based on SiO₂, scattered light is removed by apolymer coating applied to the glass fiber. Since absorbent polymercoatings with a refractive index of n>1.4 are available, it is readilypossible for noise which is caused by reflected signals and/or scatteredlight from outside the fiber to be absorbed by a polymer coating of thistype on the SiO₂ glass fiber.

Heavy metal oxide glasses which are suitable for use as fiber amplifiersusually have a refractive index of approximately n=1.9. Polymer coatingswhich have hitherto been available have always had a lower refractiveindex than heavy metal oxide glasses. Therefore, coating with polymersof this type for absorption of scattered light causes problems, since itis only possible to provide a polymer cladding with a lower refractiveindex. Any coating with a cladding made from a material with a lowerrefractive index then leads to strong, undesired reflection at theinterface between this material and the core regions or an innercladding.

Furthermore, in conventional SiO₂ amplifier fibers, there issubstantially no change in refractive index at a contact locationbetween a standard telecommunications fiber and a glass fiber of anoptical amplifier, and consequently the reflection which occurs at thetransition from a SiO₂ glass fiber amplifier to a standardcommunications glass fiber is negligible.

By contrast, the high refractive index of HMO fibers means that anycontact location with a standard SiO₂ telecommunications glass fiberleads to strong reflection at the interface between SiO₂ standard fiberand heavy metal oxide glass fiber of the optical amplifier. Since anoptical amplifier is at both outputs connected to SiO₂telecommunications glass fibers or transition fibers based on SiO₂ witha high numerical aperture, there is a considerable tendency for a laserresonator with standing light waves to form in the optical amplifier. Toprevent the latter phenomenon, it is recommended for the contactlocations in relation to the glass fibers to be designed at a defined orfinite angle. However, this in turn leads to considerable or significantreflection which is scattered into the cladding of the fiber. Therefore,scattered light which migrates through the cladding of the fiber isreflected back and forth and it is impossible to prevent scattered lightfrom reaching the central core region and penetrating into the latter.This scattered light will influence the inversion of the state of therare earth ions and leads to amplification of the noise and a drop inthe signal power(s) of the amplifier.

Outer absorbent claddings for various glass systems are known from theprior art (for example K. Itoh, et al., J. Non-Cryst. Sol, 1, pp.256-257 (1999)).

EP 1127858 describes a light-amplifying glass, the matrix glass of whichis doped with 0.01 to 10 mol % of Er, with the matrix glass necessarilycontaining 20 to 80 mol % of Bi₂O₃, 0.01 to 10 mol % of CeO₂, and atleast one of B₂O₃ or SiO₂. However, the glass fibers described in thisdocument are only provided with standard polymer coatings. The same istrue of the glasses with a high antimony oxide content described in WO99/51537.

JP 11274613 A describes a glass fiber comprising glasses with a highrefractive index, which has two glass claddings. According to thisdocument 10000 ppm of absorbent material are required. However, suchhigh levels of absorbent material influence the properties of the glassand are therefore disadvantageous.

SUMMARY OF THE INVENTION

Therefore, the object of the present invention was to provide a glassfiber comprising a matrix glass with at least one heavy metal oxide, foran optical amplifier, which allows the problems of the prior artdescribed above to be avoided. In particular, this glass fiber shouldallow the noise caused by scattered light to be minimized and thereforethe signal power of the amplifier to be increased.

This object is achieved by the embodiments of the present inventionwhich are described in the claims.

In particular, the present invention relates to a glass fiber comprisinga core, the matrix glass of which contains at least one heavy metaloxide and at least one rare earth compound, and at least two glasscladdings surrounding the core. The matrix glass has a refractive indexgreater than about 1.85, the change in the refractive index Δn from thecore to the first cladding is in a range from 0.001 to 0.08, and thefirst cladding has a lower refractive index than the core.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a diagrammatic cross-sectional view through a particularlypreferred embodiment of the glass fiber according to the invention.

FIGS. 2, 5 and 7 are respective photographic images of the cross sectionthrough glass fibers according to the invention with two glasscladdings.

FIGS. 3 and 4 diagrammatically depict preferred designs of double-cladfibers according to the invention with two or three claddings.

FIG. 6 shows a comparison of the absorbing action of iron oxide andcobalt oxide as absorbing material in a bismuth oxide-containing glasswhich has been melted under strongly oxidizing conditions.

FIGS. 8 a and 8 b show the maximum gain, calculated from Gilesparameters, for a fixed number of channels as a function of thewavelength, as well as the change in the noise as a function of thewavelength.

FIGS. 9 a and 9 b show the energy which is transmitted in each case inthe core region, in the region of the first cladding and in the regionof the second cladding, for various fiber lengths as a function of thedoping of the outer cladding.

DETAILED DESCRIPTION OF THE INVENTION

It is preferable for the core of the glass fiber according to theinvention to contain at least one heavy metal oxide which is selectedfrom oxides of Bi, Te, Se, Sb, Pb, Cd, Ga, As and/or mixed oxides and/ormixtures thereof. The matrix glass of the core particularly preferablycontains heavy metal oxides which are selected from oxides of Bi, Te, Sband/or mixtures thereof.

Furthermore, the matrix glass of the core comprises at least one dopantwhich can be excited by light. According to the invention, the matrixglass of the core contains rare earth ions as dopant. In this context, adopant is to be understood as meaning a component which is only added tothe glass in small quantities and which therefore has very littleinfluence on most of the physical properties of the glass, such as Tg,the refractive index or the softening point. However, a dopant of thistype may have a significant influence on certain properties, inparticular optical properties, such as for example the capacity foroptical stimulation.

It is preferable for the matrix glass of the core to comprise at leastone rare earth compound which is selected from compounds of Ce, Pr, Nd,Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and/or Lu. Oxides of the elementsEr, Pr, Tm, Nd and/or Dy are particularly preferred, and oxides of Erare most preferred.

If appropriate, it is also possible for Sc and/or Y compounds to bepresent in the glass according to the invention in addition to one ormore rare earth compound(s).

The rare earth compounds used as dopants are preferably what are knownas “optically active compounds”; the term “optically active compounds”is to be understood in particular as meaning compounds which lead to theglass according to the invention being capable of stimulated emissionwhen the glass is excited by a suitable pumping source.

It is also possible for at least two rare earth compounds to be used, ina total quantity of from 0.01 to 15 mol %. Glasses containing opticallyactive rare earth ions can be co-doped with optically inactive rareearth elements in order, for example, to increase the emissionlifetimes. For example, Er can be co-doped with La and/or Y. To increasethe pumping efficiency of the amplifier, it is also possible, forexample, for Er to be co-doped with further optically active rare earthcompounds, such as for example Yb. Co-doping with Gd may also beeffected in order to provide stability against crystallization.

Doping with other rare earth ions, such as for example Tm, makes itpossible to open up other wavelength regions, for example, in the caseof Tm, what is known as the S band between 1420 and 1520 nm.

Furthermore, to make more effective use of the excitation light, it ispossible to add sensitizers, such as Yb, Ho and Nd in a suitablequantity, for example 0.005 to 8 mol %.

The level of each individual rare earth compound is, for example, from0.005 to 8 mol %, preferably 0.05 to 5 mol %, based on oxide.

According to one embodiment, the matrix glass comprises both Ce and Er.

According to a further embodiment, the matrix glass contains no cerium.

According to a preferred embodiment of the present invention, the glassfiber according to the invention contains at least one Bi₂O₃ glass inthe core and/or in one or more claddings. The following compositions areparticularly preferred:

Particularly Preferred preferred components and components and Componentranges [mol %] ranges [mol %] Bi₂O₃ 10-80  30-60  SiO₂ 0-60 0.5-40  B₂O₃ 0-60 0.5-40   Al₂O₃ 0-50 0-30 Ga₂O₃ 0-50 0-20 GeO₂ 0-30 0-25 Ln₂O₃0-30 WO₃ 0-30 MoO₃ 0-30 La₂O₃ 0-30 0-15 Nb₂O₅ 0-30 0-10 Ta₂O₅ 0-15 ZrO₂0-30 TiO₂ 0-30 SnO₂ 0-40 0-30 M^(I) ₂O 0-40 0-40 M^(II)O 0-30 F and/orCl 0-10 Rare earths 0.005-8    0.05-5    (based on oxide) (based onoxide)

In the above table, M′ is at least one of Li, Na, K, Rb and Cs and M″ isat least one of Be, Mg, Ca, Sr, Ba and/or Zn. It is particularlypreferable to use Li and/or Na as M′.

FIGS. 8 a and 8 b show the gain and the noise of a doped HMOdouble-cladding fiber in accordance with the invention compared to SiO₂amplifier fibers as a function of the wavelength and the number ofchannels. To produce these diagrams, methods which are known from theprior art are used to determine the so-called Giles parameters for theamplifier fibers, and the maximum gain and the noise at a definedwavelength are then determined from the Giles parameters for a definedchannel number. It can be seen from FIG. 8 a firstly that with a setnumber of 120 channels [ch], a maximum gain of approx. 25 dB is achievedwith an amplifier fiber according to the invention, while with the samenumber of channels only a maximum gain of just below 20 dB is achievedfor a silicate-based amplifier fiber. To achieve a similar gain of 25 dBwith a silicate-based amplifier fiber, the number of channels has to bereduced from 120 to 80 channels. At the same time, with the same numberof channels the noise for the glass fiber according to the invention issignificantly lower than the noise for a silicate-based fiber. The samepicture emerges even with a further increase to 180 channels (FIG. 8 b):the fiber according to the invention has a higher maximum gain with alower noise. These FIGS. 8 a and 8 b show that broader-band transmissionat low noise is possible with the HMO glass fiber according to theinvention.

The glass fiber according to the invention, in addition to the core,also comprises at least two glass claddings which surround the core.

The cladding glasses are not subject to any particular restriction. Theypreferably have similar physical properties to the matrix glass of thecore and/or the glass of the other claddings, in particular a similarrefractive index, a similar Tg and a similar softening point. It ispreferable for the claddings to comprise substantially the samecomposition as the core, but with the compositions being modified insuch a way that the required shifts in refractive index from the core tothe first cladding and, if appropriate, from one cladding to a furthercladding are fulfilled. Furthermore, the optical properties of the coreand cladding glasses preferably differ. It is also preferable for thevarious cladding glasses to have different optical properties.

According to the invention, the term “first cladding” is to beunderstood as meaning the cladding which surrounds the core. Thecladdings are numbered in ascending order from the first claddingoutward.

According to the invention, the refractive indices mentioned are in eachcase the refractive indices of the glasses for electromagnetic radiationin the near IR region, in particular at approximately 1300 nm. Thechange in refractive index Δn from the core to the first cladding isfrom 0.001 to 0.08, particularly preferably from 0.003 to 0.04, evenmore preferably from 0.005 to 0.05, with the first cladding having alower refractive index than the core. The ratio of the refractive indexof the various claddings with respect to one another can be set asrequired using methods which are known from the prior art. To set arefractive index which is slightly higher than in the comparative glass,for example, a proportion of at least one component with a lowerrefractive index is swapped for at least one component with a higherrefractive index.

According to a first embodiment, the refractive index n_(m2) of thesecond cladding is substantially equal to or preferably higher than therefractive index n_(m1) of the first cladding. According to otherembodiments, however, it is also possible for the refractive index ofthe second cladding to be lower than that of the first cladding and fora third cladding, which has a higher refractive index than the secondcladding, to be added. Particularly preferred embodiments will be dealtwith in more detail below.

According to a first embodiment, the glass of the claddings also doesnot contain any rare earth doping, in particular any doping withoptically active rare earth compounds. According to this embodiment, theamplification and guidance of the light mode(s) preferably take place inthe core.

According to another embodiment, however, the glass of the firstcladding contains small quantities of the rare earth compound(s) used asdoping in the core. It is preferable for the first cladding to be dopedwith up to half the amount, particularly preferably up to a third of theamount, used in the core. Surprisingly, it has emerged that this measuremakes it possible to improve the signal/noise ratio of an amplifierfiber and that in this way it is also possible to improve the couplingof the amplifier fibers to SiO₂ fibers. It is assumed that with largecore radii, a more effective overlap between the signal mode and thepump mode is effected with the rare earth ions in the cladding as well.

According to a preferred embodiment of the present invention, the glassof at least one cladding, in particular of the outermost cladding,contains at least one absorbent component or an absorbent material.Absorbent components of this type which may be used include transitionmetal compounds, for example compounds of iron (in particular Fe²⁺ andFe³⁺), nickel (in particular Ni²⁺), cobalt (in particular Co²⁺),manganese (in particular Mn²⁺), copper (in particular Cu⁺ and Cu²⁺),vanadium (in particular V³⁺ and V⁴⁺), titanium (in particular Ti³⁺)and/or chromium (in particular Cr³⁺), and/or rare earth compounds. Byway of example, the doping with Fe²⁺ may amount to several 100 ppm(based on the weight ratio). The composition of the second cladding mayotherwise correspond to that of the core glass.

The level of absorbent material to be added depends on the absorption ofthe absorbent material. Levels of 5 ppm, preferably 10 ppm, may even besufficient, for example in the case of Co²⁺. It is preferable for theamount added to be at most 5000 ppm, more preferably 2000 ppm, mostpreferably at most 1000 ppm. If greater quantities of absorbent materialare added to the glass composition, the properties of the glass, such asthe crystallization properties, may be adversely affected. This istherefore not preferred.

It has been established that with certain glass compositions iron oxidesare unsuitable absorbent materials. It has been found that in particularbismuth oxide in the molten state may be reduced to form elementalbismuth, which leads to the precipitation of black metallic Bi andtherefore to a deterioration in the optical properties of the glass.Therefore, glasses which contain polyvalent heavy metal oxides, such asbismuth oxide, are preferably melted under strongly oxidizingconditions. If the glasses according to the invention are used asoptical amplifiers for the 1.5 μm band, known as the C band, theirabsorption band in the near infrared region could allow Fe²⁺ ions toserve as suitable absorbers. However, experiments have shown that 99% ofthe Fe²⁺ ions added were oxidized to form Fe³⁺ ions by the oxidizingmelting conditions. Since the absorption band of Fe³⁺ is not in therequired range, iron oxide cannot act as absorbent material in glassesproduced in this manner.

It has been found that Co²⁺ ions, which likewise have a suitableabsorption in the near infrared region, are surprisingly not convertedinto a higher oxidation state even by relatively strongly oxidizingconditions in the melt and are therefore particularly suitable for useas absorbent material in glass of this type. Therefore, it is preferablefor the outermost cladding to contain at least one preferably oxidicdivalent cobalt compound as absorbent material.

FIG. 6 compares the transmission spectrum of a bismuth oxide glasscontaining iron oxide with that of a Co²⁺-containing glass. Althoughiron has been added in the form of divalent iron (added in a quantity of1000 ppm) to the starting batch, the transmission of the glass in theregion of 1500 nm is scarcely adversely affected. The absorbent actionis therefore low. By contrast, the transmission of a glass whichcontains just 250 ppm of Co²⁺ in oxidic form has dropped to less than50% in particular in the region of 1500 nm. Therefore, cobalt oxide hasan excellent absorbent action compared to iron oxide in these glasses.

FIGS. 9 a and 9 b show the energy transmitted in each case in the core40 and the claddings 42 and 44 for two types of glass fibers accordingto the invention. FIG. 9 a shows the energy transmitted in a fiberaccording to the invention whose outer cladding 44 is doped with iron asoxidizing material. The various curves 30 to 36 correspond to differentfiber lengths. FIG. 9 a shows that with longer fiber lengths the energytransmitted in the second cladding 44 decreases in relation to theenergy transmitted in the core 40 and first cladding 42. FIG. 9 b showsthe corresponding energy transmission as a function of the radius of aglass fiber whose outer cladding 44 is doped with cobalt. The absorptioneffect of the second cladding is significantly less effective in thiscase. Scarcely any energy is transmitted in the outer cladding. Theabsorption effect is in this case independent of the fiber length.

FIGS. 3 and 4 show two particularly preferred designs of a glass fiberaccording to the invention in schematic form. These figuresdiagrammatically depict the refractive index as a function of the radiusof the glass fiber.

According to a preferred embodiment of the present invention, the coreof the glass fiber according to the invention is surrounded by preciselytwo glass claddings.

FIG. 1 shows a sectional view through a preferred embodiment of theglass fiber 1 according to the invention. The core 2 is surrounded by aninner cladding 3, which is in turn surrounded by an outer cladding 4.According to this embodiment, the outer cladding also contains anabsorbent material as described above.

FIG. 3 shows a particularly preferred design of the refractive indicesof a double-clad fiber. The region 11 is the core of the fiber, which isgenerally located approximately in the center of the fiber and is dopedwith at least one rare earth compound, the region 12 is the innercladding and has a lower refractive index than the core region 11, sothat it is ensured that the light propagating in the region of the coreis guided. The region 13 represents the second and in this case outercladding, which is primarily intended to absorb scattered light. Asshown here, the refractive index of the second cladding may be higherthan the refractive index of the core, but it is also possible for thesecond cladding to have the same refractive index as the core or a lowerrefractive index than the core. In general, an outermost cladding ofthis type has a higher refractive index than the inner cladding whichadjoins it.

According to a further embodiment of the present invention, the core ofthe glass fiber according to the invention is surrounded by preciselythree glass claddings.

FIG. 4 shows a particularly preferred design of a glass fiber accordingto the invention with three glass claddings. The region 21 representsthe core of the fiber, which is generally located in the center of theglass fiber, is doped with, for example, Er³⁺ and guides the signalmode. The inner cladding 22 may be doped with Yb³⁺. Doping of the firstcladding with, for example, Yb³⁺ in this way allows the fiber to be usedfor what is known as multimode pumping. Whereas in the case ofsingle-mode pumping light is radiated only into the core region of theamplifier fiber, and only very small lasers, which are therefore veryexpensive, can be used for this purpose, in the case of multimodepumping, light is radiated into the wider cross-sectional region of coreand, in addition, the first cladding. This radiation of light causesYb³⁺ to be excited at approx. 975 nm (²F_(7/2)→²F_(5/2)). Since Yb³⁺ isfluorescent at a similar wavelength, this fluorescence pumps the⁴I_(11/12) level of the Er³⁺ ion at approx. 980 nm. The light sourceswhich can be used for multimode pumping are significantly lessexpensive. The region of the second cladding 23, which has a lowerrefractive index than the first cladding, adjoining the first cladding22 is responsible for guiding the light which propagates in the regionof the first cladding 22, and the region of the third cladding 24 inturn serves as an outer absorbent cladding.

The glass fiber according to the invention is preferably substantiallycircular in cross section. However, the present invention alsoencompasses glass fibers which have a cross section which differs from acircular cross section.

The core of the glass fiber according to the invention generally lies inthe center of the glass fiber according to the invention, with thecladdings preferably arranged coaxially around the core. However, thepresent invention also encompasses embodiments in which the core doesnot lie in the center of the glass fiber.

Furthermore, it is preferable for the glass fiber according to theinvention to comprise precisely one core. However, according to otherembodiments it is also possible for the glass fiber according to theinvention to include a plurality of core fibers.

The glass fiber according to the invention preferably has an overallthickness of 100 to 400 μm, more preferably 100 to 200 μm. An overallthickness of approximately 125 μm is particularly preferred.

For use as an optical amplifier fiber, the core of the glass fiberaccording to the invention preferably has a diameter of from 1 to 15 μm.The thickness d_(m1) of the first cladding is preferably in the rangefrom 5 to 100 μm. The thickness d_(m2) of the second and furthercladdings is preferably in the range from 10 to 150 μm. However, forother applications it is also possible for the core and/or claddings tobe of any other desired thickness.

According to the invention, the term “core of a glass fiber” is to beunderstood as meaning the region which has been produced by the glasstechnology process and thereby differs from the cladding. By contrast, a“core region” encompasses the region in which the intensity of theoptical signal has dropped to the e^(th) part of the input intensity.

According to a further embodiment of the present invention, the glassfiber according to the invention comprises, on the outermost glasscladding, at least one coating, which comprises at least one plastic orpolymer. This outer plastic coating is used in particular tomechanically protect the glass fiber. The thickness of this plasticcoating is preferably from 2 to 400 μm. A coating thickness of less than2 μm cannot generally provide sufficient protection to the glass fiber.It is particularly preferable for the thickness to be at least 3 μm,more preferably at least 8 μm. With thicknesses of over 400 μm, itbecomes difficult to provide a uniform coating. The thickness isparticularly preferably at most 70 μm.

Any type of polymer can be used for a plastic coating of this type, solong as it bonds securely to the cladding glass. Examples of plastics ofthis type include heat-curable silicone resins, UV-curable siliconeresins, acrylic resins, epoxy resins, polyurethane resins and polyimideresins, as well as mixtures and/or blends thereof. Furthermore, thepresent invention relates to a process for producing the glass fiberaccording to the invention, in which at least two cladding glasses areformed around a core glass. This can be produced by-production processessuch as for example a “rod-in-tube” process, a multiple crucible processand extrusion processes, as well as combinations of these processes.

According to one embodiment, first of all a “preform” comprising coreand one or more claddings, is produced, this preform already having thelayer structure of the subsequent glass fiber; it can be drawn out toform a glass fiber. The thickness of a preform of this type is, forexample, from 4 to 30 mm, and its length is from 5 to 40 cm. Thispreform is drawn out to form a fiber at a suitable temperature.

In the case of a “rod-in-tube” process, a hole is drilled into acladding glass which is in the form of a strand or rod, so that atubular cladding glass is obtained. A matching rod of the core glass isintroduced into this tubular cladding glass. Furthermore, the claddingglass can also be drawn out as a tube by means of suitable shapingprocesses. By way of example, a rod of a core glass with a diameter offrom 1.0 to 1.4 mm is introduced into a tubular first cladding with adiameter of the internal hole of 1.5 mm and an external diameter of 7mm. To obtain a core surrounded with more than one cladding, it ispossible for this method to be repeated a number of times, i.e. for asecond cladding a hole is drilled into a second cladding glass in rodform, and the preform comprising core and first cladding is introducedinto the tubular second cladding. To join the interfaces, thisarrangement of core and claddings is heated, preferably to above thetransformation temperature, in order to obtain a “preform”. Ifappropriate, a preform comprising core and at least a first cladding,after it has been heated in this manner, can be drawn out to a certainextent and introduced in this drawn-out form, as a rod, into a second orfurther cladding. In the rod-in-tube process, it is also possible for ahot-formed, drawn-out rod to be fitted into a hot-formed, drawn tube.

Furthermore, a preform of this type can also be produced by what isknown as an extrusion process. In this case, a block of the core glassis placed onto a block of a cladding glass and is then heated linearlyfrom below. Along the heated line, the core glass slowly sinks into thecladding glass until it is completely surrounded by the latter.

In the case of a multiple crucible process, such as a double or triplecrucible process, a “preform” comprising a core or one or more claddingsis produced directly from the melt using nested crucibles.

According to a further embodiment of the process according to theinvention, it is also possible for a glass fiber with a diameter of, forexample, 125 μm to be produced directly, i.e. without prior productionof a preform. Triple of multiple crucible processes are used inparticular for direct fiber production.

These processes for producing a preform can be combined with one anotherin order to obtain the glass fibers according to the invention with atleast two claddings.

According to the present invention, it is particularly preferred for adouble crucible process to be used to produce a “preform” comprising thecore and the first cladding, and for the preform obtained in this way,comprising core and one cladding, to be introduced as a rod into atubular second cladding using a “rod-in-tube” process. It has emergedthat this combination on the one hand makes it possible to obtain aparticularly good interface between core and first cladding, and on theother hand allows a second and/or further cladding to be added in aneconomic way.

Furthermore, the present invention relates to an optical amplifier whichcomprises at least one glass fiber according to the invention. By way ofexample, the optical amplifier has the following structure. The incominglight signal is connected to a coupler via an optical insulator forsuppressing light reflections. Signal and pumping light are combined inthe coupler and are together introduced into the optically active fiber.The other end of the amplifier fiber is connected to the outgoing fiber.It is also possible for a filter, if appropriate with a further opticalinsulator, to be arranged here. Furthermore, it is possible for theamplifier fibers to be pumped in both directions, in which case a secondcoupler is required.

The signal light source is connected at the wave-mixing optical couplerthrough the optical insulator. Furthermore, the optical coupler isconnected to the excitation light source. Then, the optical coupler isconnected to an end of the glass fiber. The other end of the opticalglass fiber is connected to the optical insulator through the opticalcoupler for wave splitting. Each part is connected to the optical fiber.

Furthermore, the present invention comprises the use of the glass fiberaccording to the invention as optically active glass in a laserarrangement.

The present invention is explained in more detail below by means ofexamples. However, it is not restricted to these examples.

EXAMPLES Example 1

Glass compositions were produced for the core, the first cladding andthe second cladding. Table I shows the compositions of the glasses inmol %.

The core glass which had been drawn out into a strand (length 10 cm,diameter 1 mm) was sheathed with the first cladding (external diameter 7mm; internal hole diameter 1.5 mm) by means of the rod-in-tube process.The preform comprising core and first cladding was then drawn out to adiameter of 1 mm and sheathed with the outer cladding (external diameter7 mm; internal hole diameter 1.5 mm) by means of a further rod-in-tubestep.

TABLE I COMPOSITIONS OF CORE GLASS AND CLADDINGS First Second Corecladding cladding SiO₂ 14.3 14.3 14.4 B₂O₃ 28.5 28.5 21.4 Bi₂O₃ 42.442.8 50.0 Al₂O₃ 7.2 10.7 14.1 Ga₂O₃ 7.2 3.7 — Er₂O₃ 0.4 — — Fe₂O₃ — —0.1 n₁₃₀₀ ¹ 1.9931 1.984 2.047 Note: ¹Refractive index at the wavelengthof 1300 nm, measured using total reflection method on plane-parallelplates of 5 mm.

The preform obtained was drawn out to form a glass fiber with athickness of 125 μm.

FIG. 2 shows a photographic image of a cross section through a glassfiber according to the invention. Core 2 is surrounded by the firstcladding 3, which is in turn surrounded by the outer cladding 4.

Example 2

The same compositions as in example 1 were used to produce a double-cladfiber. In this case, the core was sheathed with the first cladding bymeans of a double crucible process. The core diameter and the dimensionsof the first cladding in this case corresponded to those of example 1.Then, the preform obtained in this way, comprising core and firstcladding, was drawn out to a thickness of 1.5 mm. Then, the secondcladding was formed around the drawn-out preform comprising core andfirst cladding by means of the rod-in-tube process.

The preform obtained was drawn out to form a glass fiber with athickness of 125 μm.

Optical examination revealed that in example 2 a better interface wasobtained between core and first cladding. FIG. 7 shows a photographicimage of the cross section through a fiber obtained in accordance withExample 2.

Example 3

The process described in example 1 was used to produce a double-cladfiber with core and cladding glasses based on tellurium oxide.

The preform obtained was drawn out to form a glass fiber with athickness 4 of 325 μm and a core diameter of 4.5 μm.

FIG. 5 shows a cross section through the Te double-clad fiber produced.In this case, the cross section has been etched, so that the transitionsfrom core to first cladding or second cladding are more clearly shown.

Example 4

The glass compositions shown in Table II were used to produce adouble-clad fiber. In this case, first of all a preform comprising coreand first cladding was produced using a double crucible. Then, thispreform was provided with the second cladding by means of therod-in-tube process. Next, the preform obtained was drawn out to form aglass fiber with a diameter of 125 μm.

TABLE II COMPOSITIONS OF CORE GLASS AND CLADDINGS First Second Corecladding cladding SiO₂ [mol %] 14.3 18.3 17.1 B₂O₃ [mol %] 28.5 26.526.5 Bi₂O₃ [mol %] 42.6 41.0 42.0 Al₂O₃ [mol %] 10.6 10.6 10.6 GeO₂ [mol%] 3.6 3.6 3.6 Er₂O₃ [mol %] 0.4 — — CoO [mol %] — — 0.2 n₁₃₀₀ ¹ 1.9821.964 1.978 Note: ¹Refractive index at the wavelength of 1300 nm,measured using total reflection method on plane-parallel plates of 5 mm.

Example 5

The glass compositions shown in Table III were used to produce adouble-clad fiber. In this case, first of all a preform comprising coreand first cladding was produced using a double crucible. Then, thispreform was provided with the second cladding by means of therod-in-tube process. Next, the preform obtained was drawn out to form aglass fiber with a diameter of 125 μm.

TABLE III COMPOSITIONS OF CORE GLASS AND CLADDINGS First Second Corecladding cladding Bi₂O₃ [mol %] 40.5 39.3 40.1 B₂O₃ [mol %] 30.5 28.528.4 SiO₂ [mol %] 10.3 13.9 13.0 GeO₂ [mol %] 7.6 7.6 7.6 Al₂O₃ [mol %]10.6 10.6 10.6 La₂O₃ [mol %] 0.1 0.1 0.1 Er₂O₃ [mol %] 0.4 — — CoO [mol%] — — 0.2 n₁₃₀₀ ¹ 1.973 1.959 1.969 Note: ¹Refractive index at thewavelength of 1300 nm, measured using total reflection method onplane-parallel plates of 5 mm.

1. A process for producing a glass fiber, said glass fiber comprising acore and at least two glass claddings surrounding said core, said corecomprising a matrix glass containing at least one heavy metal oxide andat least one rare earth compound, wherein the matrix glass has arefractive index of greater than about 1.85, a refractive index changeΔn from a first of the at least two glass claddings to the core is in arange from 0.001 to 0.08, and the refractive index of the first glasscladding is lower than that of the core; said process comprising thesteps of: a) making a preform, said preform comprising said core and thefirst glass cladding, by drawing from a double crucible; and b) formingat least one further glass cladding around the preform by a rod-in-tubeprocess.
 2. The process as defined in claim 1, wherein said rod-in-tubeprocess comprises drilling a through-going hole into a second claddingglass rod to form a tube of said second cladding glass, introducing saidperform comprising said core and said first glass cladding into saidtube of said second cladding glass, and drawing said tube of said secondcladding glass to form said glass fiber.
 3. The process as defined inclaim 1, wherein said at least one heavy metal oxide is at least oneoxide of Bi, Te, Se, Sb, Pb, Cd, Ga and/or As, or a mixture thereof. 4.The process as defined in claim 1, wherein the core comprises at leastBi₂O₃ and/or TeO₂ and/or Sb₂O₃.
 5. The process as defined in claim 1,wherein the at least one rare earth compound contains at least oneelement selected from the group consisting of Ce, La, Pr, Nd, Pm, Sm,Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, or a mixture thereof.
 6. Theprocess as defined in claim 1, wherein the first glass cladding containssaid at least one rare earth compound.
 7. The process as defined inclaim 1, wherein said matrix glass contains an amount of each of said atleast one rare earth compound equal to 0.05 to 5 mol %, but the firstglass cladding contains only up to one half of said amount of each ofsaid at least one rare earth compound in said matrix glass.
 8. Theprocess as defined in claim 1, wherein an outermost cladding of the atleast two glass claddings contains from 5 to 5000 ppm of at least oneabsorbent component.
 9. The process as defined in claim 8, wherein saidat least one absorbent component is at least one transition metalcompound.
 10. The process as defined in claim 9, wherein said at leastone transition metal compound comprises Co⁺².
 11. The process as definedin claim 1, wherein the index of refraction of the outermost cladding ofthe at least two glass claddings is greater than the index of refractionof the first glass cladding.
 12. The process as defined in claim 1,wherein the matrix glass has a composition, in mol % on an oxide basis,of: Bi₂O₃ 30-60  SiO₂ 0.5-40   B₂O₃ 0.5-40   Al₂O₃ 0-30 Ga₂O₃ 0-20 Ge₂O₃0-25 La₂O₃ 0-15 Nb₂O₅ 0-10 SnO₂ 0-30 MI₂O 0-40 Rare 0.5-8,  earths

wherein MI is at least one of Li, Na, K, Rb and Cs.
 13. The process asdefined in claim 1, wherein the core has a diameter of from 1 to 15 μm.14. The process as defined in claim 1, wherein the first glass claddinghas a thickness d_(m1) in a range from 5 to 100 μm.
 15. The process asdefined in claim 14, in which a second glass cladding of the at leasttwo glass claddings has a thickness (d_(m2)) in a range from 10 to 300μm.
 16. The process as defined in claim 1, wherein the glass fiber has atotal thickness of 125 μm.